Summary: 339
The amount of light coming to the eye from an object
depends on the amount of light striking the surface, and on
the proportion of light that is reflected. If a visual system only
made a single measurement of luminance, acting as a pho-
tometer, then there would be no way to distinguish a white
surface in dim light from a black surface in bright light. Yet
humans can usually do so, and this skill is known as lightness
constancy.
The constancies are central to perception. An organism
needs to know about meaningful world-properties, such as
color, size, shape, etc. These properties are not explicitly
available in the retinal image, and must be extracted by visu-
al processing. The gray shade of a surface is one such prop-
erty. To extract it, luminance information must be combined
across space. Figure 24.1 shows the well-known simultane -
ous contrast effect, which demonstrates a spatial interaction
in lightness perception. The two smaller squares are the same
shade of gray. However, the square in the dark surround
appears lighter than the square in the light surround.